Ferritic heat-resistant steel excellent in productivity and economic efficiency has been used in the major part of the high temperature part of a thermal power generation plant. For example, in a steam turbine power generation plant in which steam temperature of 600 degree-C. class or less is generally set as the steam condition, the ferritic heat-resistant steel is used in main components such as a rotor or blades of the steam turbine. However, in recent years, the efficiency of the thermal power generation plant has been actively promoted in view of environmental protection, and a steam turbine using high temperature steam of about 600 degree-C. is operated. Such a steam turbine may include many components in which required characteristics are not satisfied by the characteristics of the ferritic heat-resistant steel.
Therefore, there is a case where heat-resistant alloy or austenitic heat-resistant steel having higher temperature characteristics is used. However, the austenitic steel has a limitation in producing a large steel ingot, making it difficult for the austenitic steel to be applied to the components of the steam turbine. Therefore, a configuration is proposed in which the use of the austenitic steel is reduced in the steam turbine using a high temperature steam of 650 degree-C. or more.
There is a growing need for an increase in the thermal efficiency for reducing generation of CO2, SOx, and NOx from the viewpoint of protection of the global environment. In order to increase the plant thermal efficiency of the thermal power generation plant, an increase in the steam temperature is the most effective means, and development of a steam turbine of 700 degree-C. class is now under consideration. There are several problems to be solved in the case where the steam temperature is increased to 700 degree-C. or more. Among them, how the strengths of turbine components are guaranteed is particularly an important issue.
Conventionally, a modified heat-resistant steel is used in the turbine components such as rotors, nozzles, rotor blades, nozzle boxes (steam chambers), and steam supply pipes. However, an increase in the steam temperature to 700 degree-C. or more makes it difficult to retain a high strength of the turbine components. Thus, achievement of a new technique capable of retaining a high strength even if the conventional modified heat-resistant steel is used in the turbine components is required. In particular, the rotor assumes a high stress field by centrifugal force during operation and thus needs to be cooled so as to retain sufficient high temperature strength.
In response to the above needs, a method of cooling the rotor by distributing cooling steam inside the rotor is proposed. However, it is difficult to smoothly distributing the cooling steam inside the rotor which is a rotation field and to secure a sufficient flow rate of the cooling steam to prevent high temperature main steam from flowing into the rotor cooling area. Further, when a large amount of cooling steam is made to flow into a main steam path for cooling, the turbine efficiency may be reduced, which may in turn cause a reduction in the thermal efficiency of the entire plant.
In Japanese Patent Application Laid-Open Publication No. 63-230904 (Patent Document 1), the entire content of which is incorporated herein by reference, an apparatus that cools a rotor by blowing cooling steam to a wheel space is proposed.
However, in the example of FIG. 1 of Patent Document 1, it is not clear whether a steam pipe penetrating a casing penetrates a diaphragm or forms a cooling path different from the diaphragm. The steam pipe is directly connected to a blowing hole and it is difficult to supply cooling steam to the blowing hole at uniform pressure. In the example of FIG. 3 of Patent Document 1, a steam pipe is not provided for each stator blade, and a circumferential direction cooling steam path is provided for aiming at uniform inflow of cooling steam from the blowing hole in the dovetail portion of the rotor blade. However, sufficient uniformity of a cooling steam pressure against each blowing hole cannot be ensured.
The present invention has been made in view of the above problems, and an object thereof is to provide a steam turbine that supplies cooling steam at more uniform pressure to a blowing hole of the inner ring of a diaphragm to further increase thermal efficiency without reducing the efficiency of a steam turbine driven by high temperature steam.